The existing fleet of general aviation spark ignition piston engines, as well as new engines currently being delivered, and engines which are overhauled for use as replacements on existing aircraft, typically operate using leaded fuels, as allowed in the United States under an exemption provided by the 1990 Federal Clean Air Act Amendments. As that Act banned the use of leaded fuels for over-the-road vehicles in the Unites States, general aviation aircraft engines have become an increasingly visible source of atmospheric lead emissions. Environmental regulations and threatened regulations throughout the world have thus spurred investigations into the development and evaluation of possible alternative aviation fuels.
Most of the general aviation spark ignition piston engines in use today have been certified in the United States by the Federal Aviation Administration (FAA) for use with leaded aviation gasoline blends that meet the American National Standard No. ASTM D910-07a entitled Standard Specification for Aviation Gasolines. Under that standard, for Grade 100 fuel, 1.12 grams of lead per liter are provided in the fuel blend. In the most commonly used fuel, Grade 100LL, known as a “low lead” fuel, 0.56 grams of lead per liter are provided in the fuel blend. Both of those blends provide a minimum “knock value” lean mixture octane number of 99.6 per the ASTM D-2700 Test Method. Also, both of those blends provide a minimum “knock value” rich mixture octane number of 130, per the ASTM D 909 Test Method.
Given the regulatory environment, both in the U.S. and internationally, that seeks to require the minimization or elimination of the use of lead in general aviation aircraft reciprocating piston engines, the US FAA has been instrumental in conducting tests on various heretofore proposed formulations for low lead or no lead aviation gasolines. Their reports are publicly available through the US National Technical Information Service (NTIS), Springfield, Va. 22161. Such studies include the following reports:                (1) DOT/FAA/AR-04/25, entitled Full-Scale Engine Knock Tests of 30 Unleaded, High-Octane Blends, by David Atwood and Julian Canizales, issued by the Office of Aviation Research, Washington, D.C., in September 2004;        (2) DOT/FAA/AR-TN07/5, entitled High-Octane and Mid-Octane Detonation Performance of Leaded and Unleaded Fuels in Naturally Aspirated, Piston Spark Ignition Aircraft Engines, by David Atwood, issued by the U.S. Department of Transportation, Federal Aviation Administration, in March 2007;        (3) DOT/FAA/AR-08/40, entitled Full-Scale Engine Detonation Tests of 47 Unleaded High Octane Blends, by David Atwood, issued by the Office of Aviation Research, Washington, D.C., in September 2008; and        (4) DOT/FAA/AR-08/53, entitled Full-Scale Detonation and Power Performance Evaluation of Swift Enterprises 702 Fuel, by David Atwood, issued by the Office of Aviation Research, Washington, D.C., in January 2009.        
The September 2004 FAA report describes how over 200 blends of potential future aviation unleaded fuels were considered. Thirty of those blends, ranging in Motor Octane Number (MON) from 96.2 to 105.6 were sufficiently promising to be blended into batches and knock-tested (as determined by ASTM D-2700 standard) in a Lycoming IO-540-K aircraft engine at the FAA William J. Hughes Technical Center in Atlantic City, N.J. Components of such blends included ranges of some (or of all) of various ingredients, including super alkylate, toluene, ethyl tertiary butyl ether, meta-toluidine, ethanol, and methylcyclopentiadienyl manganese tricarbonyl (MMT), which were blended into a base fuel of either aviation alkylate or motor alkylate. Importantly, the FAA researcher reported that the performance of many of the tested blends deviated from that suggested by either their MON or by their performance number (PN).
The March 2007 FAA report compared detonation performance of mid and high octane leaded and unleaded fuels. The fuels were compared at the onset of light detonation. The fuels were tested in a naturally aspirated Lycoming IO-540-K engine and in a naturally aspirated Lycoming IO-320-B engine. For testing, the motor octane number (MON) of fuels was determined by ASTM International (ASTM) specification D2700. The supercharge rich rating was determined by the ASTM D-909 standard. In general, the testing showed that the Grade 100LL fuel (with values minimally meeting the MON and Supercharge Rating of ASTM D910-07a) significantly outperformed the matrix of tested unleaded fuels of equivalent MON, including even those with much higher ASTM Standard D-909 supercharge rich ratings, particularly as seen when operated on full scale aircraft engines rather than the laboratory test engines used to establish the ASTM Standard D-2700 MON and the D-909 rich rating performance number (PN). The March 2007 report indicates that the supercharge rich ratings do not appear to have the same significance for the matrix of unleaded fuels that were tested as they do for leaded hydrocarbon fuels. Based on the blends tested, the report clearly suggests that development of a better detonation performance unleaded aviation fuel would be desirable.
The September 2008 FAA report was a continuation of the research described in the September 2004 report. Based on the results of the potential future aviation unleaded fuel blends earlier tested, another matrix of 47 unleaded fuel blends was developed and detonation tested in a Lycoming IO-540-K aircraft engine at the FAA William J. Hughes Technical Center in Atlantic City, N.J. Components of such blends included varying ranges of “high octane components” such as aviation alkylate, super alkylate, toluene, ethyl tertiary butyl ether (ETBE), meta-toluidine, tert-butylbenzene. The blends contained iso-pentane for volatility control. Comprehensive blend formulations, by both volume fractions and mass fractions of those fuel blends were reported in Tables 2, 3, 4, and 5 of that report. The blends with a target range of 97.6 to 106.3 MON were tested against a baseline leaded reference fuel that met all specifications of ASTM D910-07a for Grade 100LL fuel with minimum MON and minimum performance number (PN) per ASTM D-909. The blends were also tested against a 100LL aircraft fuel purchased at the local airport. Here, the FAA researcher reported that none of the unleaded blends of equivalent or lower MON performed as well as the Grade 100LL fuel in the detonation tests, particularly as seen when operated on full scale engines rather than the laboratory test engines used to establish the ASTM D-2700 MON and the ASTM D-909 rich rating performance number. It was also demonstrated that increased fuel flow of the unleaded blends was required above the fuel flow required for 100LL in order to achieve equivalent detonation performance. In short, the tested blends provided less detonation protection than leaded formulations of equivalent MON, and appeared to potentially be less efficient. Importantly, the researcher again reported that using only motor octane number (MON) based on ASTM D-2700 (for knock rating, lean mixture) to predict full scale engine performance of unleaded fuels, is inadequate.
The January 2009 report provides results of tests on a high octane, bio-fuel (fermentation based) composition identified as Swift 702 fuel, from Swift Enterprises of Indiana. Swift 702 fuel was separately reported by Swift Enterprises, Inc., assignee of U.S. Patent Application Publication No. 2008/0244961 A1, published on Oct. 9, 2008, as being eighty three percent (83%) by weight of mesitylene (also known as, and hereinafter identified by the chemical name 1,3,5tri-methyl benzene), and seventeen percent (17%) by weight of iso-pentane. The FAA similarly reported that the Swift 702 fuel consisted of two pure chemical compounds. The Swift 702 fuel was reported by the FAA to have a motor octane number (MON) of 104.4, as determined by ASTM D-2700. The Swift 702 fuel was detonation tested in a Lycoming IO-540-K aircraft engine used in the tests noted in the two reports above. Also, the Swift 702 fuel was tested in a turbocharged non-intercooled Lycoming TIO-540-J2BD aircraft engine. These two engines were reported by the FAA as having been previously determined as having the highest octane requirements of engines in the active general aviation fleet. The Swift 702 fuel provided slightly better detonation performance than Grade 100LL fuel that was purchased from the local airport aviation gasoline fixed base operator. However, it did not meet the 50%, 90%, and end distillation points of the then current ASTM D910-07a specification. And, the energy content was noted as being only ninety three point six percent (93.6%) of Grade 100LL on a mass basis. Such a reduction in energy content, in conjunction with the higher fuel density, will reduce the available payload of the aircraft for a given trip of a given range. In some cases, such a reduction will be unacceptable to the operator, and may require expensive re-certification of the aircraft. Thus, it would be desirable that any replacement aviation fuel more closely meet the presently existing ASTM minimum specifications with respect to energy content per unit mass of fuel, in order to minimize any potential loss of range or payload for an aircraft using such fuels. And, it would be desirable to provide a replacement aviation fuel that minimizes the quantity of 1,3,5tri-methyl benzene that must be produced to provide sufficient unleaded fuel to the aviation marketplace, since such compound is not presently produced in commodity quantities for fuel blending, and may be more expensive, even in large scale production, than other possible unleaded aviation gasoline components.
In other work, U.S. Pat. No. 5,470,358, entitled Unleaded Aviation Gasoline, was issued Nov. 28, 1995 to Gaughan, and assigned to Exxon Research & Engineering Co.; the disclosure of that patent is incorporated herein in its entirety by this reference. The Gaughan patent discloses an unleaded aviation fuel that combines (a) an aviation gasoline base fuel having a motor octane number (MON) of 90-93, with (b) an amount of at least one aromatic amine as that is effective to boost the motor octane number (MON) of the base fuel to at least about 98. However, many high performance aircraft engines require better performing fuels, i.e. fuels that at least have the ability to run at all significant operating conditions in a manner substantially equivalent to that presently provided by at least a fuel that meets the minimum ASTM D910-07a specification for Grade 10011, if not more. An unleaded fuel blend that only provides performance equivalent to that of a 98 MON avgas on a full scale engine will likely fail at times to meet necessary engine performance requirements. Thus, it would be desirable that a fuel provide performance that meets or exceeds the minimum ASTM D910-07a specifications for Grade 100LL fuel. It would be even more desirable to provide a fuel that meets or exceeds in full scale aircraft engine testing the performance of an FBO Grade 100LL fuel having a selected MON. As discussed elsewhere herein, it is common for FBO Grade 100LL fuels to have a selected MON well in excess of the minimum ASTM D910-07a specifications for Grade 100LL fuel.
U.S. Pat. No. 6,258,134 B1, entitled High Octane Unleaded Aviation Gasolines, issued Jul. 10, 2001 to Studzinski et al., and assigned to Texaco, Inc., discloses an unleaded aviation fuel of at least 94 motor octane number (MON). The disclosure of U.S. Pat. No. 6,258,134 B1 is incorporated herein in its entirety by this reference. In an embodiment, that disclosure provides an unleaded aviation fuel having a motor octane number (MON) of at least 94, made up of the combination of (1) an unleaded alkylate base fuel having a boiling point range that is substantially wider than the range of boiling points in aviation base fuel, and having a motor octane number (MON) of at least 91, (2) an alkyl tertiary butyl ether, and (3) an aromatic amine. Yet, high performance aircraft engines require better performing fuels. Further, it would be desirable to provide an unleaded aviation fuel that avoids the use of oxygenated components, such as alcohols or ethers, especially since use of the latter class of compounds has been eliminated by governmental regulation in many countries.
In Europe, Hjelmco Oil AB of Sweden has been selling unleaded avgas of various blends, including a 91/96 motor octane number (MON) unleaded blend that may be used in 91/96 and in 80/97 octane engines. See http://www.hjelmco.com. The 91/96 UL MON blend was first produced in Finland and introduced in 1991, and is now produced in Sweden. Hjelmco now reports on the above noted website that it is considering a Bio-alkylate derived avgas in a possible replacement for existing Grade 100LL avgas. However, in so far as I am aware, they do not yet offer a product that is capable of providing adequate detonation performance in 100/130 octane aviation engines, in spite of their many years of experience in blending and providing unleaded aviation fuels.
Finally, U.S. Pat. No. 6,767,372 B2, entitled Aviation Gasoline Containing Reduced Amounts of Tetraethyl Lead, issued Jul. 27, 2004 to Barnes et al, and assigned to Chevron U.S.A. Inc., discloses an unleaded aviation fuel of at least 94 motor octane number (MON). The disclosure of U.S. Pat. No. 6,767,372 B2 is incorporated herein in its entirety by this reference. In an embodiment, that disclosure provides an unleaded aviation fuel having, measured by volume, (a) about twenty percent (20%) to about eighty percent (80%) of iso-octane, (b) about five percent (5%) to about eighteen percent (18%) of toluene, (c) about one percent (1%) to about twenty percent (20%) of C4 to C5 paraffins, (d) greater than zero (0) to about one (1) ml of tetraethyl lead per gallon of the aviation gasoline composition, and (e) the balance of the composition being light alkylate produced in an alkylation unit using hydrogen fluoride or H2SO4 as a catalyst. In an embodiment, that aviation gasoline is described as being substantially free of ether compounds, such as methyl tertiary butyl ether (MTBE) or ethyl tertiary butyl ether (ETBE) or the like. However, the Barnes et al patent does not describe whether or not there is any possibility within the otherwise described ingredients to completely eliminate the use of tetraethyl lead. And, although it teaches reduced lead compositions in an aviation fuel, it does not provide specific suggestions as to possible formulations using the components described therein that might tend to further minimize or eliminate the use of tetraethyl lead in order to meet or exceed performance standards for presently existing for Grade 100LL aviation fuel.
Thus, in spite of the extensive testing and evaluation by the FAA and by others of various candidate unleaded aviation fuel blends, and other work as noted in the above described patent literature, there still remains an as yet unmet need for an unleaded aviation fuel blend that can be readily used in the existing general aviation piston engine aircraft fleet as a “drop in substitute”. Such a fuel, particularly a fuel that is essentially transparent in functionality to the aircraft engine during various flight operations as compared with existing Grade 100LL fuels, and which could be mixed in the aircraft fuel tank in a random manner with existing Grade 100LL fuel formulations, would assist in the reduction or phase out of existing lead containing aviation gasolines. That is because rather than requiring a simultaneous wholesale and widespread switch in aviation fuel availability, existing fuel systems could accommodate and provide a new unleaded aviation fuel as it becomes locally available from suppliers. And, aircraft crews would not need to be concerned with whether previously existing 100LL fuel or a new unleaded aviation fuel blend were available at any particular airfield. Further, it would be advantageous if a new aviation fuel were available that could be utilized with little or no mechanical alterations or replacements of existing aircraft engines or aircraft system adjustments, and which could be used with little or no additional certification or other regulatory changes from the aircraft owner or operator standpoint. And, such a fuel would be of benefit to aircraft engine manufacturers and to aircraft manufacturing companies, as a fuel having such characteristics should enable them to avoid the need for extensive redesigns of equipment, testing, and recertification that might be required if an unleaded aviation fuel with less desirable performance characteristics were selected for widespread use. It would also be especially advantageous if in an embodiment, such a new unleaded aviation fuel, rather than having substantially less than existing energy content for use by the aircraft, would provide as much or more energy per unit volume of fuel tank capacity, i.e. British Thermal Units (BTU's) per gallon, as existing Grade 100LL fuels. In such a manner, it would be particularly advantageous if a new unleaded aviation fuel could be used to take full advantage of the existing mechanical design components with respect to mass flow of air into the engine, and materials of construction utilized in the fuel system, and be capable of operating without knock or detonation at rich and lean air fuel ratio conditions, with existing compression ratios, with full rated power output, in a stable and highly efficient manner in all flight operating conditions, including high power cruise conditions with lean air-fuel mixtures.
Moreover, it would be advantageous to provide a new aircraft fuel that may be produced and distributed as a substitute for, and in the same manner as, existing petroleum feedstock aircraft fuels, using existing refinery production systems and fuel distribution systems. It would be even more useful if such a replacement aircraft fuel were provided that meets the ASTM D910-07a specification for detonation margins and further, either meets the remaining ASTM D910-07a Table 1 requirements or which only exhibits deviations from those requirements of a nature and to an extent that are not operationally significant to the pilot and the aircraft while completely eliminating the use of lead additives.
It would also be advantageous to accomplish such goals while providing a replacement aircraft fuel suitable for “drop-in” substitution, fully fungible with existing Grade 100LL aviation gasoline, in order to minimize the extent, complexity, and cost of any recertification efforts of the high performance, high-octane fuel powered engines found in existing general aviation aircraft. As used herein, the term “drop-in” substitution is directed to a fuel that meets aircraft engine performance requirements from an operational standpoint, and can be used transparently, from the operational standpoint (including fueling of and holding in the fuel tank, holding and processing in the fuel systems of an aircraft during storage and during operation, and consumed by combustion during operation of the aircraft engine, and producing environmentally acceptable products of combustion). As such, a “drop-in” fuel as described herein may or may not meet all of the current ASTM D910-07a specifications requirements (or a future/then current later generation similar fuel specification), except for the absence of lead. Unofficially, in some aviation fuels industry circles, such usage—i.e. meeting performance requirements but not strictly meeting ASTM or other specifications—might otherwise be known as having the capability of a “quasi-drop-in” fuel—i.e. a fuel that meets performance requirements but not strictly all of the applicable ASTM D910-07a specifications. In any event, it would be very helpful to the general aviation piston engine user community to have available a fuel which could be placed in the aircraft tanks and used without regard to changes in mechanical components or aircraft performance, and which will therefore minimize or eliminate regulatory paperwork. It would be even more helpful, and quite advantageous, for a new unleaded aviation fuel to be made available that meets such objectives, and that also can be used without change in existing operational manuals or procedures.